The invention relates to a control circuit for a power supply associated with an electronic flash unit. More particularly, the invention relates to such control circuit which utilizes a timer circuit of a photographic camera with which the electronic flash unit is associated to automatically deactivate a voltage booster of a power supply for the unit whenever the electronic flash is left in its operative condition over a given time interval, thereby suppressing the dissipation of a battery or cell which is used as a power supply.
As is well recognized, an ordinary electronic flash unit which is internally housed within or mounted on a photographic camera utilizes a dry cell of a low electromotive force as a power supply. For this reason, the supply voltage is stepped up by a voltage booster formed by a DC-DC converter before it is supplied to a flashlight illumination circuit. In the flashlight illumination circuit, the booster voltage charges a main capacitor of an increased capacity, the voltage across which is then applied to a flash discharge tube to activate it in response to a shutter release operation.
An electronic flash unit includes a power switch which is connected between a power cell and a voltage booster and which is turned on to initiate the operation of the booster whenever the electronic flash unit is to be used. The purpose of such arrangement is to avoid a power dissipation of cell which would occur sooner if the booster circuit was normally left in its operative condition. However, a user of a photographic camera with which the electronic flash unit is associated is likely to forget to operate the power switch. If the switch is left on, the cell will be rapidly exhausted, preventing an effective operation of the electronic flash unit. Also, the power switch may be inadvertently turned on thereby permitting a wasteful dissipation of the cell as a result of an oversight.
Recent photographic cameras contain a variety of electrical controls, and are designed to achieve a saving in the power dissipation by utilizing a timer circuit which operates to disconnect an internally housed cell, even though the main switch remains closed, to prevent a wasteful dissipation of the cell unless a given operation is tried within a given time interval after the main switch has been closed. By way of example, a display of exposure information within the viewfinder by light emitting diodes will be automatically interrupted unless a next scheduled operation is tried within a given time interval determined by a timer circuit. In other words, recent photographic cameras containing a variety of electrical controls include a timer circuit which produces time limit signals of given lengths for the purpose of suppressing the power dissipation of the supply cell.